The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to a method and apparatus system for formation of a gradient coil to allow for increased gradient slew rates and reduced peripheral nerve stimulation. It will be appreciated, however, that the-invention is also amenable to other like applications.
MR tomography is a known technique for acquiring images of the inside of the body of a living examination subject. To this end, rapidly switched magnetic gradient fields of high amplitude, which are generated by gradient coils, are superimposed on a static basic magnetic field. In the process of generating MR images, stimulations can be triggered in living examination subjects by the switching of the gradient fields. The gradient fields that influence the examination subject are characterized by a magnetic flux density that varies over time. The time-varying magnetic field generates eddy or induction currents in the examination subject. Their nature depends primarily on the shape and size of the microscopic structures. Due to electromagnetic interaction with tissue in the examination subject, these currents influence physiological currents, for instance potentials at cells. All cells have a resting potential. At resting potential, all membrane currents of a cell are in balance. When the membrane potential is depolarized by an additional membrane current, which is introduced into the cell by an outside influence, for example, this causes a potential change, known as an action potential. The actuating potential for an action potential is called a threshold. At the threshold, the balance of the membrane currents changes. Additional currents temporarily appear, which depolarize the membrane. An action potential is accompanied by an action. Thus, for example, each contraction of a muscle fiber is accompanied by an action potential in the muscle fiber, and each reaction of a sensory cell to a sensory stimulus is relayed by action potentials. Accordingly, due to the triggering of action potentials, switched gradient fields can lead to stimulations that are experienced as uncomfortable by the examination subject.
Due to the abovementioned physiological effects on the patient, constraints are placed on maximum gradient amplitudes and switching speeds (slew rate) for the gradient fields. As stated above, time-varying magnetic fields induce currents in conductive materials and rapidly changing magnetic field gradients can induce currents in a patient being imaged. Under some circumstances, these induced currents can stimulate nerves, a phenomenon known as peripheral nerve stimulation (PNST). Therefore, every MRI employed for human patients must conform to one or more magnetic field amplitude and rate of change limitations in accordance with FDA regulations. Thus, current MRI systems, therefore, assume the worst possible circumstances and limit the gradient slew rates amplitudes accordingly.
Most physiological limits placed on the gradient field rate of change are not a single fixed value. Instead, the limit changes as a function of the “transition time” (i.e. the time interval over which the change in gradient field occurs). The reason for allowing higher rates of change (i.e. dB/dt) as the transition time decreases is related to the fact that the electrical sensitivity of neurons decrease with increasing frequency. J. P. Reilly of the Johns Hopkins University Applied Physics Lab has modeled the response of nerve cells and produced an equation predicting the dB/dt threshold for peripheral (PNST) and cardiac nerve stimulation as a function of dB/dt and pulse duration. It should be noted that cardiac stimulation occurs at dB/dt levels about 10 times that of PNST, therefore, a wide margin of safety is realized. The Reilly PNST equation, known as the “Reilly Curve,” is the basis for the FDA physiologic limits on dB/dt.
All gradient coil designs intended for human use will have a physiologic limit given by the Reilly equation. The slew rate, which gives the limit, however, will depend on the effective length of the coil. The physiologic slew rate limit is determined by dividing the Reilly limit by the effective length of the gradient coil, L. Note that, the effective length L is not necessarily the true length of the coil. The effective coil length is the ratio of the maximum field strength in milliTesla (mT) found within the gradient coil divided by the applied gradient strength (mT/m). While the effective coil length has units of length, it does not relate to any physical dimension within the coil. It should not be confused with the distance from the iso-center of the gradient coil to the location of maximum field variation. Maximum field strength is defined as the vector sum of all three components of field produced by the gradient coil axis.
There are known methods for predicting these nerve stimulations. One of these methods for monitoring stimulations is based on the so-called “dB/dt model”. In this method the values, which occur in an MR tomography process, of the time variation of magnetic flux density of gradient fields (dB/dt values) are checked and monitored. The maximum allowable dB/dt values result from a stimulation study with the corresponding gradient coils, or from limit values that have been strictly prescribed by regulatory authorities such as the FDA.
The triggering of stimulations for a selected gradient configuration depends, basically, on the type of measurement sequence. It is necessary to distinguish between sequences known as conventional measuring sequences and sequences known as rapid measuring sequences. Conventional measuring sequences usually demand a high linearity of the gradient fields within a definite linearity volume, for instance 5% in a linearity volume of 40 to 50 cm given moderate gradient strengths of 10 to 20 milliTesla/meter (mT/m) and switching times of approximately 1 millisecond. However, for rapid measuring sequences, high gradients, for instance 20 to 40 mT/m, are switched very rapidly (switching times approx. 100 to 500 microseconds). The time-varying magnetic flux density of the gradient fields induces electrical currents in the examination subject, which can trigger nerve stimulations of the subject. With faster time variations, that is, faster switching times and larger values of magnetic flux density of gradient fields, the induced currents are greater, and the likelihood of nerve stimulations increases. The largest values in absolute terms are attained at the margins and outside the linearity volumes, where the maximum field deviation or excursion also occurs. Given defined requirements on the size of the gradients and the switching time, the field deviation, and thus the risk of stimulation, can be reduced by using a gradient coil with a smaller linearity volume. Thus, in rapid measuring sequences, the linearity volume of typically 40 to 50 cm drops to 20 cm, for example. A gradient coil with the above-described characteristics for rapid measuring sequences typically is not suitable for conventional whole-body applications, but rather for rapid MR imaging techniques such as described in U.S. Pat. No. 4,165,479 and what are known as turbo-spin methods.
The UK patent application GB 2,295,020 describes a modular gradient coil system that unites, in one coil body, a gradient coil for rapid measuring sequences and an activatable gradient coil for conventional measuring sequences. The gradient coil for rapid measuring sequences has a small linearity volume and allows rapid switching of gradient fields with large gradients. In the joint operation of the two coils, the gradient coil system has a large linearity volume for conventional measuring sequences with slowly switched gradient fields and given small gradients. This has the disadvantage that, with the selection of a rapid or conventional measuring sequence, an imaging region is defined corresponding to the appertaining linearity volume. The imaging region for rapid measuring sequences is always a definite small sub-region, which is strictly prescribed by the coil arrangement, of the larger imaging region for conventional measuring sequences, with the midpoint of the two imaging regions being identical. To pick up MR images with rapid measuring sequences for an imaging region extending over the imaging region for conventional measuring sequences, the examination subject would have to be moved in all three directions in space. Due to the geometry of the MR tomography device, however, it is only possible to shift the examination subject in one direction.
Furthermore, U.S. Pat. No. 5,311,135 teaches a gradient coil for a magnetic resonance device which has four saddle-shaped coils, each of which has first and second terminal points respectively at the beginning and end of its conductor, as well as at least one tapping point between the terminal points. The arrangement also includes a switching mechanism, so that each of the coils can be supplied with current either between the terminal points or between the first terminal point and the tapping point. In this way, at least two different linearity volumes of the gradient coils can be set, for instance corresponding to a size of a region that is being imaged.
A gradient coil with at least two independently controllable portions, with multiple control states for generating a gradient field for imaging multiple regions is described in U.S. Pat. No. 6,418,336 to Kimmlingen et al. In this patent, by controlling gradient fields for at least two imaging sub-regions, with neither of the two regions being a subset of the other, it is possible to pick up MR images for a larger aggregate imaging area, which derives at least from the sum of the two imaging sub-regions, using rapid, high-resolution measuring sequences without triggering stimulations.
Configurations of gradient coils that employ extra “twin” or excess coils are costly and complicated. Moreover, such coils utilize additional space limiting space for cooling and shaping to address other design parameters. Therefore, there is a need in the art for a gradient coil design that addresses current regulatory requirements for field strength and slew rate requirements without excessive complication and cost.